Blackwidow Futa + Gwen Stacy

Blackwidow Futa in an alternate universe from the Avengers, comes with Tony Stark to recruits Peter Parker for the Avengers. Gwen Stacy was at the house however, and Black Widow takes notice.
She discreetly slips a note to Gwen and they meet up later that night and fuck at Gwen's house.

Simple.

Later we can tie Mary Jane Watson in and they can have a Futa-Female-Female threesome, Blackwidow fucking both barely-legal girls who are also both virgins.

I love Emma Stone and her as Gwen Stacey is the only reason to watch The Amazing Spiderman films. I'm not into lesbianism but I'd read about Emma Stone opening a can of peaches if it was in the paper. I'd definitely write it if your version of Stacey was the film one. Scarlet Johansen is very popular too so I'm sure there's an audience for it. Good luck with it.